The invention relates to power units comprising an internal combustion engine having variable-volume combustion chambers supercharged by a turbocompressor having a turbine which receives the engine exhaust gases and comprising by-pass pipe means which may be permanently open during operation of the engine, designed to convey the air not drawn by the engine from the compressor to the turbine with a pressure drop which, if appreciable, is substantially independent of the flow rate and increases with the compressor outlet pressure. An auxiliary combustion chamber is disposed upstream of the turbine and receives at least part of the air which has travelled through the by-pass pipe.
The invention applies to engines, having rotors as well as to an engine having reciprocating pistons, and to engines ignited by sparking as well as by compression. It is specially advantageous, however, in the case of an installation comprising a motor constituting a displacement machine (i.e., more particularly a four-stroke engine as opposed to a two-stroke engine). In this case also, the invention is of particular advantage in the case of power units comprising a four-stroke engine having a low volumetric ratio (below 12 and even possibly below 6), supercharged by a turbine-compressor unit whose compressor has a high compression ratio (up to 6 or even more) and operating near its surge line so that its efficiency is also high.
The term "turbocompressor" or "turbine-compressor unit" is to be construed as covering the case where there are a number of turbine units and/or stages or where there are a number of compressor units, the air being cooled if required between successive compressor units.
In the power units of the aforementioned kind which is described in French Patent Specification 2,179,310 and in the corresponding U.S. Pat. No. 3,988,894 the by-pass pipe is provided with throttle means which subject the air flowing from the compressor to the turbine to a pressure drop which is substantially independent of the flow rate and increases in linear proportion with the compressor outlet pressure (and usually represents 5 to 15 % of the last-mentioned pressure). The by-pass pipe enables the turbocompressor to operate like a gas turbine with high efficiency, near its surge line.
On the other hand, some of these power units have a limitation.
The supercharging turbocompressor is selected to match the engine when the latter operates at the rated point, i.e., at maximum power (maximum torque and maximum speed). The matching is generally such that, at the rated point, the compressor supplies the flow rate of air drawn by the engine plus an additional 5 or 15% of the flow rate taken in by the engine, the additional flow being used for:
Maintaining an air flow in the by-pass pipe so as to maintain the pressure difference between the compressor and the turbine,
And/or supplying the oxygen needed for a pilot burner in the auxiliary combustion chamber, if such a chamber is provided,
And/or feeding circuits for cooling hot engine components (e.g. the exhaust manifold, the spark-plugs if any, etc.),
And/or supplying reserve air for covering variations in the ambient conditions or progressive clogging of the air filters in use.
For decreasing the manufacture and operation costs, the compressor is typically selected to meet but not to exceed the aforementioned requirements.
If now the air intaken by the engine is at a substantially constant temperature, the line representing engine operation at constant speed (the flow-rate/pressure characteristic) is approximately a straight line passing through the origin, at least in the case of a four-stroke engine constituting a volumetric machine. On the other hand, the operating point of the turbocompressor moves along a curve which is concave towards the pressure axis and which extends through (a) a rated matching point corresponding to the rated pressure and a flow rate between 5 and 15% higher than that taken by the engine and (b) a point representing a zero flow rate for a pressure ratio equal to unity.
Consequently, the two characteristics inevitably intersect, whatever the engine speed (the only effect of which is a decrease in the slope of the engine flow-rate/pressure characteristic when the engine speed is lower).
If the pressure ratio is allowed to drop to and below the point of intersection, the air flow in the branch pipe will first stop and then reverse, thus preventing normal operation of the engine and, more particularly, any acceleration without load, as we shall see hereinafter.
One obvious method of overcoming the difficulty would be to inject fuel into the auxiliary combustion chamber at a rate controlled by regulating means preventing the supercharging pressure from falling below a value slightly above that corresponding to the intersection of the curves. In most engines, however, this method is unsatisfactory since it substantially increases the total fuel consumption when the engine is idling or under low load.
It is an object of the invention to provide an improved power unit of the above-mentioned type, wherein the aforementioned disadvantage is at least partially overcome.
It is another object of the invention to take advantage of the fact that in power units as disclosed in the aforesaid U.S. Pat. No. 3,988,894 the pressure difference upstream and downstream of the by-pass pipe is independent of the flow rate through the branch pipe (the difference being the same as between the compressor outlet and the turbine inlet) so as to solve a problem which is common to all supercharged engines having a low compression ratio and is rendered more acute if the intake is throttled; and which consists in starting and operating the engine under idling and low-power conditions when the ambient temperature is very low.
According to an aspect of the invention, there is provided an internal combustion engine having variable volume combustion chambers, a supercharging turbocompressor unit having a compressor and a turbine, the inlet of said turbine being connected to receive the exhaust gas of said engine,
pipe means connecting the outlet of the compressor to the intake of the engine,
by-pass conduit means having an inlet and an outlet connected to flow the air delivered by said compressor and not drawn by the engine to the inlet of said turbine with a pressure loss which is substantially independent of the ratio of the flow rate traversing said by-pass conduit means to the air flow delivered by the compressor and increases with the output pressure of the compressor,
an auxiliary combustion chamber connected to receive at least part of the air which circulates along said by-pass conduit means and the exhaust gas of said engine and having an output connected to the inlet of said turbine,
adjustable air flow throttling means diposed in said pipe means downstream of the junction of said bypass conduit means with said pipe means,
control means having means responsive to the load of the power unit and operatively associated with said throttling means for limiting the rate of air flow taken by the engine when the unit delivers a low amount of power and maintaining said rate of air flow at a value lower than the air flow delivered by the compressor.
Advantageously the automatic actuating means are responsive to engine operating parameters so as to maintain a constriction such that the flow rate through the by-pass pipe is sufficient to establish a well-defined pressure difference between the turbine inlet and the compressor outlet and to provide the oxygen necessary for fuel burning in the auxiliary chamber, and also so as to ensure that the engine exhaust temperature does not exceed a permissible limit.
The power unit preferably comprises recycling pipe means communicating with the output of the auxiliary combustion chamber and with the intake of said engine and non-return valve means in said pipe means which open when the pressure in the intake is lower than the pressure at the output of the auxiliary combustion chamber for recycling combustion gas from said output to said intake.
Thus, the engine intakes a fraction of its exhaust gases and of the combustion gases from the auxiliary chamber together with the air required for fuel combustion during start-up at low temperature.
Supercharged Diesel engines provided with a throttle valve at the engine intake are known (U.S. Pat. No. 2,633,698 Nettel). In these engines, however, the throttle valve fulfills a completely different purpose from that contemplated by the invention, and does not give the same result. More specifically, the only purpose of the valve in such prior art engines is to render starting the engine at low ambient temperature easier; when the flow of air from the compressor is stopped by throttling the valve, the air is heated by compression before it enters the engine (e.g., see col. 1, lines 52-55, and col. 2, lines 1-6).
In such prior art supercharged Diesel engines, a pipe can also be provide for recycling exhaust gases, in which case the pipe must be provided with a throttle valve for adjusting the ratio between the exhaust-gas flow rate and the rate of air arriving through a wide open duct. The throttle valve is usually manually controlled and it can be only very approximately adjusted. Starting at very low temperature remains difficult, either because too much exhaust gas is recycled so that combustion becomes incomplete and the engine may choke and stall, or because not enough is recycled, thus preventing starting. Furthermore, the throttle valve is immersed in high-temperature gases and therefore rapidly deteriorates.
It should be noted that the invention does not consist simply in changing the position of a throttle valve disposed downstream of the compressor so as to facilitate starting of supercharged engines in accordance with a prior-art feature, for the same purpose as in the case of an engine according to French specification 2,179,310. The invention consists in a combination of throttle means and means for controlling them and which come into action when the engine is idling or slightly loaded so as to fulfil a function which was not described (and had no reason to be described) in the prior art.
A fundamental difference between the results sought by the two methods is also clear from the fact that, apparently, the only result of a prior art valve is in a power unit comprising a permanently open pipe is to aggravate the problem which the valve is intended to solve in U.S. Pat. No. 2,633,698, i.e., when the load is low, the throttle means tend to direct air down the by-pass pipe.
In the power unit according to the invention, the difficulty is overcome by combining a by-pass pipe producing a well-defined pressure drop and wide-open recycling means, so as directly to regulate, not the ratio of the two flow rates, but the flow rate of air to the engine, which is maintained at a value such that there is no exhaust overload. In the by-pass pipe there are two pressure levels which may be called "upstream" and "downstream" and are both determined by the upstream pressure only. When the throttle means at the engine intake provide an air flow cross-sectional area which is insufficient for the intake manifold pressure to be greater than the downstream level, recycling occurs and restores the downstream level such that a hot atmosphere, at the same pressure as downstream of the engine, prevails in the intake manifold. Since the "downstream" pressure level is not influenced by the extent to which the throttle means are opened, the flow rate of fresh air to the engine is dependent only on the flow cross-sectional area provided by the throttle means. Thus, the cross-sectional area can be adjusted to fulfil a given condition, e.g. maintaining the engine intake temperature at a constant value, without a complete feed back loop.
The recycling means are typically arranged to recycle the engine exhaust gases and gases from the auxiliary chamber to the intake. Then the regulating means can be simplified and the gases taken at the auxiliary chamber outlet contain little or no unburnt hydrocarbons, unlike the exhaust gases. Finally, the combustion gases are much hotter, and can therefore be recycled at smaller mass flow rates.
The recycling means can consist of a pipe having a large cross-section (so as not to produce an appreciable pressure drop) and provided with non-return means (e.g. a check valve having a light spring basing it toward its seat). In that case, the system for regulating the throttle means can be designed to operate only when the engine load falls below a value which is just above the value at which the auxiliary chamber comes into action so as to maintain the compressor outlet pressure at a threshold level necessary for self ignition of the engine.